When a projection terminal is placed, it is difficult for a projection centerline of the projection terminal to be perpendicular to a projection screen and directly opposite to a center point of the projection screen. Thus, when projection light from the projection terminal to the projection screen is shorter, diffusion of the light on the projection screen is smaller; while when the projection light from the projection terminal to the projection screen is longer, diffusion of the light on the projection screen is larger, and widths of upper and lower sides of an image projected on the projection screen will be different, i.e., a trapezoidal image will appear on the projection screen.
In the existing art, the above problem can be solved by the keystone correction technology. The principle of the keystone correction technology is to adjust and compensate for a shape of an image before projection by using electronic devices and an interpolation algorithm, such that the image projected on the projection screen becomes a rectangular image. In the existing keystone correction technology, the working principle of the automatic keystone correction technology includes the following steps. An included angle between the projection centerline of the projection terminal and a projection plane (projection screen) is detected firstly, for example, an accelerometer is provided in the projection terminal and used to detect the included angle between the projection centerline of the projection terminal and the projection plane (projection screen). After the included angle between the projection centerline of the projection terminal and the projection plane is obtained, trapezoidal distortion of the image is automatically corrected according to the included angle between the projection centerline of the projection terminal and the projection plane.
However, in practical applications, it is found that the effect of the keystone correction is very poor in some cases.